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They can be but very much depends on the property in my experience. We had a one bed garden flat a couple of years ago and not surprisingly it was mostly first time buyers who were interested and were then terrified when they were made aware. I think the vendor ended up taking an offer but I really can't remember how much now.
Then had a house this year which attracted a different type of buyer where it wasn't quite the same reaction. That house actually achieved the asking price as well.
As an estate agent I've come across knotweed a lot. As you say it's not the red flag it used to be and it certainly wouldn't stop me from buying a property that has it.